The present invention relates to a blank plate for forming multi-color fluorescent planes and a method for forming multi-color fluorescent planes and, more particularly, to a blank plate for forming multi-color fluorescent planes for plasma display panels (hereinafter PDPs for short), etc. and a method for forming such multi-color fluorescent planes.
Conventionally, the direct current (DC) and alternate current (AC) types of PDPs have been known in the art. FIG. 8 illustrates the DC type of PDP which includes flat front and back plates 51 and 52 arranged opposite and parallel to each other, both being formed of glass. The back plate 52 is fixedly provided on its front side with a cell barrier 53 perpendicular thereto to define a cell 54, and the front and back plates 51 and 52 are spaced away from each other at a suitable interval by that cell barrier 53. The front plate 51 is provided on its back side with an anode 55, and the back plate 52 is provided on its front side with a cathode 56 perpendicular to said anode 55. On both sides of the anode 55 there are formed adjacent fluorescent planes 57.
In the above conventional DC type of PDP, an electric field is applied between the anode 55 and the cathode 56 to give rise to discharge in the individual cells 54 defined by the front and back plates 51 and 52 and the cell barrier 53. Ultraviolet rays generated by such a discharge then causes the fluorescent planes 57 to fluoresce, allowing a viewer 58 to make visual observation of the light trasmitting through the front plate 51.
Turning now to FIG. 9, there is shown the conventional AC type of PDP, which includes flat front and back plates 61 and 62 of glass arranged in opposition and parallel to each other, and in which the back plate 62 is fixedly provided on its front side with a cell barrier 63 perpendicular thereto to define a cell and the front and back plates 61 and 62 are spaced away from each other at a suitable interval by that cell barrier 63. The back plate 62 is provided on its front side with two electrodes 64 and 65 perpendicular to each other through a dielectric layer 66 with additional provision of a conductive layer 67 and a protective layer 68. The front plate 61 is provided on its back side with a fluorescent plane 69.
In the above conventional AC type of PDP, an alternate voltage is applied between the two electrodes 64 and 65, thereby giving rise to discharge in the individual cells defined by the front and back plates 61 and 62 and the cell barrier 63. Ultraviolet rays generated by such a discharge then cause the fluorescent plane to fluoresce, permitting a viewer 70 to make visual observation of the light transmitting through the front plate 61.
In some cases, the DC type of PDPs may include a mask 59 for preventing the reflection of external light, which comprises a light-shielding portion, so as to avoid the reflection of external light, as illustrated in FIG. 10. It is noted that, in FIG. 10, parts similar to those shown in FIG. 8 are shown by like reference numerals.
The fluorescent planes of the DC or AC type of PDPs of such structures are formed by coating a fluorophore-containing photosensitive slurry on the back sides of the front plates and, then, exposing it to light with the use of a photomask corresponding to a fluorescent plane pattern, followed by development and firing.
More specifically, an anode 72 is formed on one side of a transparent substrate 71 formed of glass, etc., as illustrated in FIG. 11(a). The anode 72 may be defined by a transparent electrode. However, when a panel is of increased size or is used for TV displays driven at high speeds, a gold (Au) paste is usually screen-printed and fired on the substrate to form an anode. This is because the transparent electrode has an increased resistance value. Then, a slurry liquid 73 containing a fluorophore of a given color is coated on the anode 72, as illustrated in FIG. 11(b). Thereafter, a photomask 74 is disposed on the side of the substrate 71 opposite to the side having the fluorophore-containing slurry liquid 73 coated thereon, as illustrated in FIG. 11(c), followed by exposure to given light, e.g., ultraviolet light. Subsequent development gives fluorescent planes on both sides of the anode 72, as illustrated in FIG. 11(d).
The slurry coat is thus exposed to light from the side opposite to the anode 72 through the substrate 71 for the following reason. In short, when it is to be exposed to light from the side having the anode 72 formed thereon, the photomask having its portion corresponding to the electrode shielded against light is located on the fluorophore slurry liquid 73 of FIG. 11(b). In this case, if there is only a slight deviation of the position of the photomask, the fluorescent planes then become asymmetric with respect to the anode 72. Further, the fluorescent planes may deteriorate or malfunction, since they are positioned such that they cover the anode 72. According to the above process, on the other hand, no fluorescent plane is formed on the anode 72, since the anode 72 itself functions as a mask.
It is appreciated that mixtures containing, e.g., a fluorophore, polyvinyl alcohol (PVA) and a diazonium salt may be used as the photosensitive slurry liquid, optionally with antifoamers and surface active agents.
In full-color panels, fluorophores emitting three fluorescences of red, green and blue have to be selectively arranged. The steps of coating of a photosensitive slurry containing each emissive fluorophore, exposure to light with the use of a photomask corresponding to the predetermined arrangement and development are repeated by the number corresponding to the kinds (usually three) of fluorophores to form multi-color fluorescent planes. Used in this case is a process wherein provided are photomasks including light-transmitting segments corresponding to the respective color arrangements, which are equal in number to the emissive colors, and the light-transmitting segments are regulated such that they are in agreement with the respective color arrangements for the purpose of exposure to light.
In such a conventional process for forming multi-color fluorescent planes, however, it is required that the photomasks corresponding in number to the emissive colors be in precise coincidence with the respective color arrangements. In the actual steps of forming multi-color fluorescent planes, it is still very difficult to bring the photomasks in complete coincidence with the respective color arrangements with high accuracy. For that reason, the following problems arise:
(1) First of all, when a plurality of photomasks are used for each emissive color, only a slight deviation of the total pitch, etc. of the mask patterns gives rise to a variation in the relative positions of the fluorescent plane corresponding to each cell and the cell structure, thus posing such problems that any precise colors of the panel are not obtained, or uneven colors are emitted over the entire surface of the panel.
(2) Secondly, extreme difficulty is encountered in achieving the position alignment of the photomasks with the front plate. In order to bring the photomasks corresponding to the emissive colors in coincidence with the respective color arrangements, the photomasks or the front plate are moved by a moving device such as a stepping motor, while monitored through a camera. Since inherent limitations are placed upon the accuracy of the camera or moving device, however, it is indeed impossible to accurately set its X, Y and .theta. (a tilt) directions. As is the case with the above (1), a slight deviation of position alignment makes it impossible to obtain any faithful reproducibility of colors and any uniformity of colors emitted over the entire surface of the panel.
(3) Repeated precise position alignment of the full-surface plate with the photomasks results in complicated steps and a lowering of productivity.
(4) The plasma display panels are expected to be used with substrates of considerable diagonal sizes from 9 to 14 inches for office automation terminals to 20 to 80 inches for flat TVs. However, to make photomasks corresponding to emissive colors with more precise total dimensions, pitches, etc. offer a problem difficult to solve in view of the thermal expansion of the material used and setting-up of working atmosphere. For instance, if soda glass is used as the material for a substrate of a diagonal size of 40 inches, there is then an about 9-.mu.m diagonal deviation as a whole, given a temperature change of 1.degree. C., since the soda glass has a coefficient of thermal expansion of 92.times.10.sup.-7 /.degree.C. Hence, stringent temperature control is needed.